Transparent photochromic glasses or phototropic glasses, as they have been variously termed, have been marketed commercially for about 25 years. Such glasses darken (change color) when exposed to actinic radiation, commonly ultraviolet radiation, and fade back to their original color, i.e., they return to their original optical transmission, when withdrawn from the actinic radiation. The basic disclosure of such glasses is found in U.S. Pat. No. 3,208,860 (Armistead et al.), which patent is directed generally to silicate base glass compositions wherein crystals of silver chloride and/or silver bromide and/or silver iodide confer the photochromic behavior to the glass. The preferred base glasses had compositions within the alkali metal aluminoborosilicate field.
A multitude of patents have been granted subsequently, those patents being drawn to expressly-delineated areas of silicate glass compositions which exhibited exceptional or unusual photochromic properties and/or other remarkable physical and/or chemical behavior. And, in general, the glass compositions encompassed within those patents have also been based in the alkali metal aluminoborosilicate system with silver chloride and/or silver bromide crystals providing the desired photochromism.
The most common application of photochromic glasses has been as ophthalmic lenses and non-prescription sunglasses. The most widely-marketed photochromic glasses for ophthalmic lenses have been produced by Corning Incorporated, Corning, New York, under the trademarks PHOTOGRAY EXTRA and PHOTOBROWN EXTRA. The glass fabricated into ophthalmic lenses marketed under the trademark PHOTOGRAY EXTRA is essentially colorless in the original or fully faded state, and darkens to a gray coloration when subjected to actinic (ultraviolet) radiation. That glass is included within U.S. Pat. No. 4,190,451 (Hares et al.). The glass formed into ophthalmic lenses marketed under the trademark PHOTOBROWN EXTRA is likewise essentially colorless in the original or fully faded state, but darkens to a warm brown coloration when exposed to actinic (ultraviolet) radiation. That glass composition is encompassed within U.S. Pat. No. 4,251,278 (Hares).
The wearer of photochromic eyeglasses not only wishes to have the glass demonstrate good photochromic properties, i.e., the glass quickly darkens to transmissions below about 40% at room temperature, preferably below 30%, and fades quickly, i.e., the glass fades at least 25 percentage points, preferably at least 30 percentage points, after a five minute withdrawal from the actinic radiation at room temperature, but also wishes to have the glass be cosmetically appealing. This concern with cosmetic appearance is especially prevalent in the sunglass market where all colors and shades of colors can be found.
Various means have been proposed for imparting color to photochromic glasses. For example, U.S. Pat. No. 4,240,836 (Borrelli et al.) discloses modifying the surface of silver halide-containing glass articles by heat treating such glass articles under critically-defined conditions. The heat treatment causes the reduction of silver ions to metallic silver in contact with silver halide microcrystals in a region very near the surface of the glass, with the observed color being determined by the geometric form and arrangement of metallic silver on those microcrystals. Such glass articles can exhibit orange, red, purple, or blue surface coloration in the undarkened state.
Various well-recognized glass colorants have been added to the base photochromic glass composition to impart permanent coloration thereto. For example, U.S. Pat. No. 4,018,965 (Kerko et al.) describes the addition of colorants such as the rare earth metal oxides Er.sub.2 O.sub.3, Pr.sub.2 O.sub.3, Ho.sub.2 O.sub.3 and/or the transition metal oxides CoO, NiO, and Cr.sub.2 O.sub.3 to produce a variety of permanent tints in the glass.
As was noted above, U.S. Pat. No. 4,251,278 discloses the production of photochromic glasses which are essentially colorless in the undarkened state, but which demonstrate a warm brown coloration when exposed to actinic radiation. Those glasses contained crystals of silver chloride and/or silver bromide and/or silver iodide to impart photochromism to the glass. The warm brown color was developed in the glass due to the presence of 1-10 parts per million (ppm) of Pd and/or Au. The patent also describes transparent photochromic glasses exhibiting a brown coloration in the undarkened state and a deeper warm brown coloration in the darkened state. Those glasses also contained crystals of silver chloride and/or silver bromide and/or silver iodide to produce photochromism therein, but included more than 10 ppm Pd and/or Au. Finally, the patent further discloses transparent, silver halide-containing photochromic glasses containing 1-50 ppm Pd plus up to 1% total of 0-0.5% CoO, 0-1% NiO, and 0-1% Cr.sub.2 O.sub.3 and/or up to 5% total of at least one rare earth metal oxide selected from the group of Er.sub.2 O.sub.3, Pr.sub.2 O.sub.3, Ho.sub.2 O.sub.3, and Nd.sub.2 O.sub.3, wherein the glass exhibits a particular tint in the undarkened state, that tint being dependent upon the colorant selected, but which tint is complemented with a warm brown hue in the darkened state. As an example, the patent cites the use of Er.sub.2 O.sub.3, but without specifying the amount included, to impart a permanent pink color to the glass in the undarkened state which, upon the inclusion of Pd and/or Au in amounts greater than 10 ppm, will demonstrate a brown coloration with a warmer tint being complemented with the pink from the Er.sub.2 O.sub.3.
U.S. Pat. No. 4,746,633 (Mazeau et al.) describes transparent photochromic glasses that in a thickness of 2 mm can darken to a comfort range, i.e., an optical transmission of about 35-60%, when exposed to actinic radiation at room temperature, and which will fade back to a transmission greater than 70% after five minutes' withdrawal from the actinic radiation. The base glasses therefor consisted essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 46-60 MgO 0-3.5 B.sub.2 O.sub.3 16-28 CaO 0-6 Al.sub.2 O.sub.3 4-11 SrO 0-6 ZrO.sub.2 2-6 BaO 0-6 Al.sub.2 O.sub.3 + ZrO.sub.2 6-13 MgO + CaO + SrO + BaO 0-7 Li.sub.2 O 2-5 P.sub.2 O.sub.5 0-5 Na.sub.2 O 0-4 TiO.sub.2 0-3. K.sub.2 O 2.5-9 ______________________________________
The "photochromic elements", i.e., Ag, Br, Cl, and CuO, consisted essentially, as analyzed in weight percent, of
______________________________________ Ag 0.13-0.18 Cl 0.16-0.34 Br 0.08-0.14 CuO 0.001-0.004 ______________________________________
The patent notes that additions of up to about 1% total of classic glass colorants such as CeO, Er.sub.2 O.sub.3, MnO, and NiO can be incorporated to tint the glass, and up to about 1% SnO or up to 4 ppm Pd can be added to provide a brown coloration in the darkened state.
Surveys and focus group studies have indicated a substantial market for transparent photochromic glasses that, in thicknesses of 2 mm, would darken to a dark range upon exposure to actinic radiation at room temperature, that would fade rapidly when removed from the actinic radiation, and that would exhibit a pink coloration of a definite hue in the darkened state. Thus, those studies showed a consumer desire for glasses which, in the darkened state, would exhibit a color as defined by the trichromatic coordinates determined through the trichromatic colorimetric system of the C.I.E. of 1931 utilizing Illuminant C as the light source between the ranges of
x&gt;0.3500 and &lt;0.3700 and PA1 y&gt;0.3250 and &lt;3450.